Generally, the magnetron for generation of a high frequency energy is a kind of diode vacuum tube, as shown in FIG. 1, in which a serial type cathode (hereinafter called as "filament") 4 having a cathode portion axially provided at its center and an anode 1 provided around its circumference.
On the other hand, an upper yoke 6a and a lower yoke 6b are provided for applying a magnetic flux between the filament 4 and an anode 1, and a magnetic circuit is formed which is provided in turn with magnet 7 and magnetic pole 8 respectively to the top and bottom surfaces of each yoke 6a,6b, and an antenna 9 for discharging the high frequency energy transmitted from the anode 1 to exterior (cavity), an antenna seal 10, an antenna ceramic 11 and an antenna cap 12 are provided.
And, heat radiating plate 3 for radiating the heat generated by the collision of thermions (thermally charged electrons) at the anode vane 1a is provided, and a choke coil 13 for blocking that unnecessary high frequency component generated at the operating space 5 and high voltage capacitor 14 are made so as to be protected by a filter box 15.
Accordingly, the above-described magnetron executes cycloidal motion by receiving forces of electric field in which thermions emitted from the filament 4 are applied between the anode vane 1a and the filament 4 and the magnetic flux applied to an operating space 5 by magnetic pole 8 on the magnetic circuit, and according to this the thermion experienced acceleration becomes to generate the high frequency energy, therefore the anode vane 1a receives this energy.
A magnetron disclosed in Japanese laid open Utility Model publication official gazette No. Sho-52-109439 is a form as shown in FIG. 2, which comprises a filament 4 made of coil shape which emits thermions, a upper end shield 16 for supporting a top end of the filament 4, a lower end shield 17 for supporting a bottom end of the filament 4, a side lead 19 for supporting the lower end shield 17 and conducting the filament 4, a center lead 18 for supporting the upper end shield 16 and conducting the filament 4, and a getter 24 fixed by welding on top surface of the upper end shield 16.
Aforementioned filament 4 is made of usually thorium-tungsten, the getter 24 is made of zirconium, and the end shields 16,17 are made of high melting point metal of molybdenum.
On the other hand, a spacer 20 for preventing a vibration is provided at the middle of the both leads 18,19, and the ends of both leads are supported at an insulating ceramic 21 so as to be connected with filament terminals 22, respectively.
And, a seal 23 is adhered for making these to a vacuum bulb body, and the getter 24 at the top of the upper end shield 16 is for absorbing a generating gas of the vacuum bulb body.
Accordingly, when a power is supplied to the filament, a voltage is applied through the both end shields 16,17 to the filament 4, and the filament 4 is heated to high temperature and a large quantity of thermions 25 are emitted from the surface of the filament 4 as shown in FIG. 3, which is attracted toward the anode vane 1a having a positive (+) potential as shown by arrows so as to generate an oscillation by a mutual operation with magnetic field.
On the other hand, in order to prevent a separation of the thermions along an axial direction, a downwardly extended bent portion 16b is formed at both ends of flat portion 16a of the upper end shield 16.
However, in such conventional magnetron, the thermions emitted from the top end, e.g., a neighboring portion of the upper end shield 16 can not be attracted to the entire anode portion, and collide to the internal surface of the bent portion 16b formed at the upper end shield 16 and thereafter reflected and come out, and accordingly secondary electrons a,b by the colliding energy becomes generated. Therefore, movement of the thermions are scattered by a disturbance of the electric field pattern, and it is impossible for 100% of the thermions emitted from the filament 4 to flow to the anode portion. Accordingly, the thermions contributing or causing the oscillation become decreased and an oscillation efficiency of the magnetron has been reduced.
Since a material of the upper shield is molybdenum which does not have good plasticity or elasticity characteristics, the upper end shield is of a complicated form requiring much expense and time in manufacture.